The present invention relates to a novel molding resin composition based on a polyvinyl alcohol, referred to as PVA hereinafter, for shaping of a film or sheet or, more particularly, to a polyvinyl alcohol-based molding resin composition compounded with a specific additive ingredient and suitable for melt-molding in a substantially anhydrous condition.
Polyvinyl alcohol as a full or partial saponification product of a poly(vinyl ester) is a useful resinous material for shaping of films and sheets by melt-molding. Different from other thermoplastic resins, however, polyvinyl alcohol resins are subject to thermal decomposition at a temperature lower than the melting point thereof so that polyvinyl alcohol resins are usually melt-molded into films and sheets by the method of extrusion molding or casting with water as the plasticizer. Such a molding method for a hydrous resin requires addition of a large volume of water to the resin so that it is essential that the molding machine is operated under exact temperature control or, in particular, the temperature of the extrusion die is kept not to exceed 100.degree. C. in order to prevent bubble formation in the extruded material while the film or sheet as extruded must be continuously dried and subjected to a heat treatment. Therefore, the molding process is necessarily troublesome and the costs for the preparation of films or sheets are very high.
Various proposals and attempts have been made heretofore to solve the above mentioned problems in the molding method of PVAs. For example, Japanese Patent Publication 37-12538 proposes to convert a PVA to a sulfate of PVA by the addition of sulfamic acid in order to improve the compatibility of PVA with water. Japanese Patent Publication 42-18813 discloses a method in which a PVA is compounded with a plasticizer which is an ester of phosphoric acid with an alcohol, glycol ether and amide compound. Various additives are proposed in order to prevent coloration of a PVA by heat in the course of thermal forming of the PVA including sodium sulfonate Salts of dibasic fatty acid ester (Japanese Patent Publication 43-3975), mercaptothiazole or imidazole compounds (Japanese Patent Publication 45-7691), carboxylic acids (Japanese Patent Publication 46-37664) and alkali metal salts of phosphoric acid (Japanese Patent Publication 47-24265). Japanese Patent Kokai 49-131240 teaches admixture of a propionic acid adduct of glycerin with a PVA of low saponification degree of 35 to 60% by moles. Japanese Patent Kokai 49-120946 teaches admixture of a glycerin adduct of phthalic acid with a PVA of low saponification degree of 70 to 97% by moles. Japanese Patent Kokai 51-88544 teaches that coloration of a PVA can be prevented by the admixture of phosphoric acid or pyrophosphoric acid.
No sufficient improvements, however, can be obtained in each of these prior art methods for preventing coloration of PVAs in a dynamic thermal forming process accompanied by heat evolution in PVAs such as extrusion molding and calendering.
Another important factor influencing the workability of a PVA in thermal forming is the releasability of the resin from metal molds and other metallic surfaces of molding machines. In this regard, a method is proposed in Japanese Patent Kokai 4-202535, 5-98038 and 5-96617 according to which the workability in melt-molding of a PVA can be improved by compounding the PVA with a salt of an alkali metal or alkaline earth metal. Japanese Patent Kokai 5-163369 teaches that an improvement can be accomplished in the mold-releasability and plate-out behavior of a PVA resin in the process of thermal forming by using an alkyl polyoxyethyleneadduct of dimethyl ammonium chloride as a surface active agent. Further, Japanese Patent Kokai 5-194812 proposes to improve the die-releasability of a PVA resin in extrusion molding by the use of an aqueous silicone emulsion. Though not ineffective for the improvement of the workability in thermal forming of a PVA resin, however, these methods provide no or little improvement for the thermal stability of the resin to cause coloration so that the methods have low practical applicability.
Besides, Japanese Patent Publication 44-28588 proposes a method for the melt-molding of a PVA resin in an anhydrous condition by the admixture of a polyhydric alcohol. This method, however, can hardly be practiced as an industrial method due to the difficulty in accomplishing long-run continuous operation of the molding process as a consequence of accumulation of resinous residue called scorch on the surfaces of the screws, extrusion dies and the like of the molding machine to greatly affect the smoothness of the surface or appearance of the products. In addition, proposals have been made in Japanese Patent Publication 42-26930 for the melt-molding of a modified PVA obtained by the saponification of a polyvinyl acetate in the presence of a polyhydric alcohol, in Japanese Patent Kokai 50-22049 for the melt-molding of a PVA compounded with an ester of a polyhydric alcohol with a condensation product of an aromatic carboxylic acid and a condensed-ring hydrocarbon compound and in Japanese Patent Publications 53-24975 and 56-49733 for the melt-molding of a resin composition consisting of an .alpha.-olefin-modified PVA and a thermoplastic resin in an anhydrous condition but no fully anhydrous condition of molding can be accomplished in these methods.
Japanese Patent Kokai 3-203932 further proposes a method of melt-molding of a copolymeric PVA containing polyoxyalkylene groups but this method is not free from difficulties in respect of the high costs for the copolymerization of vinyl alcohol, vinyl ester, oxyalkylene allyl ether and the like, the relatively low yield of the copolymer and recovery and recycling of the solvent used for the copolymerization.